Applicants' invention relates to circuits for improving the transient response of an electric power supply, and in particular to a circuit having a higher-than-desired voltage output from which a current is sourced to suppress the transient during a voltage drop on a main output.
An important consideration in electric power supply design for circuits and systems having fast and large amplitude load changes is the voltage deviation when the power supply output current changes rapidly. Frequently, the design of the supply's output filter makes it difficult for the output current to respond quickly enough. The reason for this is that the output ripple must be kept low, which is inconsistent with a fast load transient response.
One way to improve the transient response is to increase the supply's switching frequency, reduce the output filters, and increase the bandwidth of the feedback loop. Nevertheless, the performance achievable by this technique is not enough to meet the requirements of recently developed computer architectures and other uses, in which active power management and clock rate control results in huge power changes in a few microseconds. The output voltage must still be maintained within a narrow tolerance band.
For example, the model PKU4110PI is a 100-watt converter that is commercially available from Ericsson Components AB. A 25-ampere (25-A) load current change having a rise time less than ten microseconds (10 .mu.s) produces a drop of more than 200 millivolts (mV) on the output of the PKU4110PI. Although this is a transient response which is excellent, it is still not good enough for some demanding applications. It is no longer uncommon to require a voltage drop of less than 50 mV for such a load transient.
One way to improve transient response performance that has been tried is the use of an additional output. U.S. Pat. No. 4,074,182 to Weischedel describes a d.c. power supply that comprises two voltage regulators connected in parallel to a common load. One of the regulators is a spare that supplies the load automatically if the output of the other regulator decreases. This arrangement purportedly can minimize load transients.
U.S. Pat. No. 4,622,629 to Glennon describes a power supply that has a first rectifier and a supplemental rectifier, which supplies power when the first rectifier's output drops.
U.S. Pat. No. 5,408,172 to Tanimoto et al. describes a power supply having a driving circuit that is turned on when the current demanded by a load increases and causes a drop in the load's power supply voltage.
These previous methods and circuits have not successfully addressed all of the problems and have not provided all of the desirable features.